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The patch that introduced the SessionWorker removed a couple of telemetry measurements:
https://hg.mozilla.org/mozilla-central/rev/242b07a68acf
We should re-add them. Those are the timestamps that aren't recorded anymore:
FX_SESSION_RESTORE_READ_FILE_MS
FX_SESSION_RESTORE_BACKUP_FILE_MS (the time to move the file)
FX_SESSION_RESTORE_WRITE_FILE_MS should probably be moved to the Worker as well to get a real timing without any event queue hiccups.
As the operations are running on the worker synchronously, we can use Date.now() to measure the running time and return those numbers to the chrome process via messages. The chrome process can then submit those values to Telemetry like here:
http://mxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/TelemetryStopwatch.jsm#108

Created attachment 783435[details][diff][review]
Patch - Restore telemetry measurements
The worker now returns a telemetry object which can contain measurements taken inside. It is the callers responsibilities to record the measurements.
FX_SESSION_RESTORE_READ_FILE_MS and FX_SESSION_RESTORE_BACKUP_FILE_MS have been restored, and FX_SESSION_RESTORE_WRITE_FILE_MS now measures just the write inside the worker.
Try: https://tbpl.mozilla.org/?tree=Try&rev=bca5db918ebd